Minimum-Energy Conical Intersections by Compressed Multistate Pair-Density Functional Theory

Abstract

Compressed multistate pair-density functional theory (CMS-PDFT) is a multistate version of multiconfiguration pair-density functional theory that can capture the correct topology of coupled potential energy surfaces (PESs) around conical intersections. In this work, we develop interstate coupling vectors (ISCs) for CMS-PDFT in the OpenMolcas and PySCF/mrh electronic structure packages. Yet, the main focus of this work is using ISCs to calculate minimum-energy conical intersections (MECIs) by CMS-PDFT. This is performed using the projected constrained optimization method in OpenMolcas, which uses ISCs to restrain the iterations to the conical intersection seam. We optimize the S 1 /S 0 MECIs for ethylene, butadiene, and benzene and show that CMS-PDFT gives smooth PESs in the vicinities of the MECIs. Furthermore, the CMS-PDFT MECIs are in good agreement with the MECI calculated by the more expensive XMS-CASPT2 method.

Optimizing Conical Intersections without Explicit Use of Non-Adiabatic Couplings

“…We note that, while density functional theory (DFT) based methods do not provide wave functions, recent developments for generalizing DFT to include a multiconfiguration character can alleviate this difficulty. This is for example the case for the compressed multistate pair-DFT method, where interstate coupling vectors can explicitly be calculated . In wave function-based electronic structure methods, the analytic evaluation of the DC can become computationally expensive or difficult to implement and, thus, it may not be available for numerically complex electronic structure methods.…”